To the best of our knowledge, this is the first large prospective cohort study to explore the potential relationship between ADL disability, frailty, and depression in the middle-aged and elderly Chinese population based on CHARLS. Overall, we found that a positive association between ADL (including BADL and IADL) disability and the risk of depressive symptoms existed, and that frailty was associated with increased risk of depressive symptoms. Furthermore, the association was partially mediated through frailty. Besides, the results of the sensitivity analyses showed no interaction between ADL disability and the stratification variables, and the results were robust.
Previous studies explored an association between ADL disability and depression, but results remained controversial. A cross-sectional study of Chinese older adults[32] showed that the association between ADL disability and depression was significant in both urban and rural older adults. In Chinese middle-aged and older adults, ADL disability was associated not only with an increased risk of depressive symptoms in themselves but even in spouses without ADL disability[33]. However, the study by Barry et al. pointed to no significant association between older adults with depression and ADL functioning[34]. In addition, previous studies have been inconsistent in their identification of a causal relationship between ADL disability and depression[35–37]. Therefore, we demonstrated the causal association between ADL disability and the onset of depressive symptoms in the prospective cohort study.
The results of our study confirmed that ADL disability is positively associated with an increased incidence of depressive symptoms, with higher ADL scores being associated with a higher incidence of depressive symptoms. Physical functioning is known to decline with age. Physical decline is a key challenge to the ability of older adults to care for themselves, and it is difficult to perform many daily activities independently as physical functioning declines[38]. ADL is essential to maintaining independence in older adults. Therefore, the direct mechanism by which ADL disability leads to an increased incidence of depressive symptoms may be due to the negative experiences caused by the inability to perform activities of daily living[39]. As a result of their reduced physical condition, subjects with lower levels of ADL scores are unable to independently perform one or more activities of daily living, which can directly lead to a reduced quality of life and thus more feelings of depression. The indirect effect of ADL disability on depressive symptoms may be due to psychological stress triggered by limitations in one's mobility. Low and declining ability to perform ADLs may constitute a significant stressor for elderly individuals. The limitations in daily activities and physical functioning can lead to a loss of independence and dependence on others for ADLs which may result in lower self-esteem, loss of self-control, and impaired personal and professional fulfillment for these individuals[39, 40]. This psychological distress caused by limitations in outside activities, such as feelings of loss, frustration with slow recovery, and the inability to cope with dependency, can greatly affect the lives of older adults and lead to depressive symptoms and sadness[41]. In addition, studies have shown that elderly individuals who have lost their independence are at higher risk of social isolation and have higher rates of morbidity and mortality[34, 42, 43]. Depression is therefore to be expected.
The association between ADL disability, frailty, and depression has been analyzed in previous studies[44, 45], with a higher prevalence of ADL disability and frailty observed in depressed patients, both of which are risk factors for depression. Studies have also explored the associations between ADL disability and depression, ADL disability and frailty[46], and frailty and depression[47], respectively. There is a correlation between all three and that frailty may play an important role in the pathway linking the two. For the first time, our study extended previous research and evaluated the possible mediating role of frailty between ADL disability and depression. We found that frailty significantly mediated the association between ADL disability and the onset of depressive symptoms. High scores on ADLs caused high levels of frailty, which increased the risk of depressive symptoms.
In both aging research and practice, frailty is a key concept and a strong predictor of a range of health problems. We showed that ADL disability was significantly associated with frailty. The result was in line with existing knowledge that ADL is important influencing factor of frailty[48]. This implies that hypoactive somatic functions contribute to the onset and development of frailty. Somatic function reflects the strength and functional status of muscles throughout the body, which is closely related to sarcopenia and is an integral part of the somatic component of frailty[49]. In addition, reduced somatic function has been associated with several comorbidities, and cognitive deficits, which exacerbate the progression of frailty[50]. A positive association between frailty and depressive symptoms was also noted in our study, which is consistent with previous findings. A meta-analysis noted that compared to non-frail individuals, frail individuals had increased odds of depression (OR = 4.42,95% CI 2.66–7.35) and that frailty increased the risk of depression[14]. As a syndrome common to older adults, frailty is affected by a variety of factors, such as pain, mobility and balance problems, and poor endurance[51]. All of these factors can lead to a poorer sense of experiencing life, which can lead to depression. Depression may also be caused by a state of frailty due to reduced gait speed and physical activity, or by an increase in sedentary behavior, risk of falls, weight loss, and malnutrition, causing the persistence of their typical emotional symptoms, including sadness, lack of pleasure and helplessness[52]. In addition, there may be common pathophysiological pathways between depression and frailty, such as cerebrovascular disease and chronic inflammation. Subclinical vascular disease that causes high signals in prefrontal white matter in patients with late-life depression has long been considered a key factor in the prevalence of frailty[53]. A growing body of evidence also supports a positive correlation between inflammatory cytokines and frailty[54], such as interleukin 6 (IL 6) and C-reactive protein, which are associated with decreased muscle mass and strength. These two inflammatory factors, in turn, are elevated in patients with late-life depression, negatively affecting central dopaminergic function, which may contribute to fatigue and depressed mood[55, 56]. Thus, inflammation may be one of the common pathogenic mechanisms of depression and frailty.
This study had some advantages. First, this is a prospective study with long-term follow-up in the middle-aged and elderly Chinese population based on the CHARLS database, which was representative of the health status of the Chinese population. Second, our research's most significant advantage is that it's the first mediation study to evaluate how frailty affects the relationship between ADL disability and depressive symptoms. The results may be useful in creating focused intervention plans to treat and prevent depression. However, there are still some limitations. Firstly, the CES-D-10 was employed to assess the number of self-reported depressions in the previous week, which might introduce recollection bias. Secondly, all missing values were excluded, which may have reduced the representativeness of the sample. Future studies would require further analysis of the effect of the ADL disability and frailty on depression.